If you guessed "Nicotine, of course," you'd be right. But now researchers have zeroed in on the exact brain cells that nicotine triggers to cut appetite and body fat.

It turns out that nicotine activates different kinds of brain-cell switches, or receptors, with very different effects, says study leader Marina Picciotto, PhD, of Yale University.

"We found that nicotine reduced eating and body fat through receptors implicated in nicotine aversion and withdrawal rather than reward and reinforcement," Picciotto says in a news release.

That's an important distinction, says Nora D. Volkow, MD, director of the National Institute on Drug Abuse.

"These results indicate that medications that specifically target this pathway could alleviate nicotine withdrawal as well as reduce the risk of overeating during smoking cessation," Volkow says in a news release. Volkow was not involved in the study.

Such medications may be a long time in coming, as the Picciotto study was in mice and not in humans. But it's a very important finding, as the brain receptors identified have implications not only for smoking cessation but for other forms of addiction, as well as for weight loss, says study researcher Mariella De Biasi, PhD, assistant director of the Center on Addiction, Learning, and Memory at Baylor College of Medicine.

The study "is not only important for the people that are trying to quit smoking, but the results provide a target for the development of drugs that might help to control obesity and related metabolic disorders," De Biasi says in a news release."